The electrocardiogram (ECG or EKG) provides a graphic representation of the electrical depolarization and repolarization processes of the cardiac muscle, as "viewed" from the body surface. The amplitude of these electrical potential differences between various points on the body is measured in millivolts (mV) and their duration in seconds. The ECG can provide information on heart rate, rhythm, and intracardiac conduction; it may also reveal evidence of specific chamber enlargement, myocardial disease or ischemia, pericardial disease, certain electrolyte imbalances, and some drug toxicities. But note that although the ECG is a valuable part of the cardiac evaluation, it cannot determine if congestive heart failure is present, or (in itself) predict whether an animal will survive procedures requiring anesthesia, nor can it provide much information on the strength (or even presence) of cardiac contractions.

Sinus rhythm is the normal cardiac rhythm, described above. The P waves are positive in the caudal leads (II and aVF), the P-Q intervals are consistent and the R-R intervals occur regularly, with less than 10% variation in timing. Normally, the QRS complexes are narrow and upright in leads II and aVF; however, if an intraventricular conduction disturbance or ventricular enlargement pattern is present, they may be wide and abnormally shaped.

Sinus bradycardia is a rhythm that originates in the sinus node and is conducted normally but has too slow a rate, while sinus tachycardia also originates in the sinus node and is conducted normally but is too rapid.

Sinus arrhythmia is characterized by a cyclical slowing and speeding of the sinus rate, most commonly associated with respiration. The rate tends to increase on inspiration and decrease with expiration because of changes in vagal tone. Often, there is an accompanying change in P wave configuration (wandering pacemaker) with the P waves becoming taller and spiked during inspiration and flatter in expiration. Marked sinus arrhythmia occurs in some animals with chronic pulmonary disease. Sinus arrhythmia is a normal rhythm variation. It is commonly seen in dogs, but not often in the clinical setting in normal cats. However, cats frequently have sinus arrhythmia when relaxed or sleeping.

Sinus arrest is a cessation of sinus node activity lasting at least twice as long as the patient's longest expected R-R interval. The resulting pause in heart rate is interrupted by either an escape beat or resumption of sinus activity. Fainting or weakness may result during these pauses.

Conduction blocks in the major ventricular conduction system also disturb the normal activation process and result in altered QRS configurations. The portion of the ventricles served by the diseased bundle branch is activated late and slowly, resulting in widening of the QRS with the terminal forces oriented toward the area of delayed activation.

Rhythm Disturbances

Impulses originating from outside the sinus node are abnormal and create an arrhythmia (dysrhythmia). Abnormal or ectopic impulses are described based on their site of origin (atrial, junctional, supraventricular, ventricular). They are also characterized by timing, that is, whether they occur earlier than the next expected sinus impulse (premature) or whether they occur late (escape), as a rescue mechanism. Abnormal premature impulses (complexes) may occur singly or in multiples. Groups of three or more comprise an episode of tachycardia; bouts of tachycardia may be brief (paroxysmal tachycardia) or quite prolonged (sustained tachycardia). A bigeminal pattern occurs when each normal QRS is followed by a premature complex; the origin of the premature complexes determines whether the rhythm is atrial or ventricular bigeminy.

Supraventricular (atrial, junctional) premature complexes originate above the AV node, in either the atrium or the AV junctional (near the AV node) area; however, since they are conducted through the ventricles in the normal manner, their QRS configuration is normal (unless an intraventricular conduction disturbance is also present). Atrial premature complexes are preceded by an abnormal P wave (either positive, negative or biphasic).

Ventricular premature complexes (VPCs or PVCs) originate below the AV node and do not activate the ventricles by the normal pathway; therefore, they have an abnormal ECG configuration. Ventricular ectopic complexes are also wider than the normal QRS complexes because of their slower conduction through ventricular muscle. When the configuration of VPCs or tachycardia in a patient is consistent, the complexes are described as being uniform or unifocal. When the VPCs occurring in an individual have differing configurations, they are said to be multiform. Increased electrical instability of the heart is thought to accompany multiform VPCs or tachycardia. Ventricular tachycardia defines a rapid series of VPCs (greater than 100 beats/minute in the dog, for example). The R-R interval is usually regular, although some variation is not uncommon. Sinus P waves may be seen superimposed on or between the ventricular complexes; they are unrelated to the VPCs because the AV node and/or ventricles are in the refractory period (physiologic AV dissociation).

Atrial fibrillation ("delirium cordis") is a common arrhythmia characterized by rapid, chaotic electrical activation of the atria. There are no P waves on the ECG; rather, the baseline usually shows irregular undulations (fibrillation waves). Since there is no organized electrical activity, meaningful atrial contraction is absent. The AV node, being constantly bombarded with these disorganized electrical impulses, conducts as many as possible to the ventricles. The (ventricular) heart rate is, therefore, determined by how many impulses the AV node can conduct. Atrial fibrillation results in an irregular heart rhythm, which is usually quite rapid. Most often, the QRS complexes appear normal in configuration, since the normal intraventricular conduction pathway is used. Atrial fibrillation tends to be a consequence of significant atrial disease and enlargement in small animals.

Atrio-ventricular (AV) conduction blocks may result from therapy with certain drugs, high vagal tone, and organic disease of the AV node and/or ventricular conduction system. AV blocks are also called "Heart Blocks."

First degree (1°) AV block: conduction of an impulse from the atria into the ventricles is prolonged, although all impulses are conducted.

Second degree (2°) AV block: intermittent AV conduction; some P waves are not followed by a QRS complex. When many P waves are not conducted, the patient has "high-grade" 2° heart block. There are two subtypes of 2° block:

Mobitz type I (Wenckebach) is characterized by progressive prolongation of the P-R interval until a nonconducted P wave occurs; it is frequently associated with disorders within the AV node itself and/or high vagal tone.

Mobitz type II 2° block is characterized by uniform P-R intervals preceding the blocked impulse, and is thought to be more frequently associated with disease lower in the AV conduction system (e.g., His bundle or major bundle branches).

Third degree (3°) or complete AV block: no sinus (or supraventricular) impulses are conducted into the ventricles. Often, there is a regular sinus rhythm or sinus arrhythmia evident; however, the P waves are not related to the QRS complexes, which result from a (usually) regular ventricular escape rhythm.